A pipeline that only ever sees 0x00..seq will green-light a decoder that does not parse. FlyBy’s simulator can emit protocol-aware payloads: layouts the rest of the stack is supposed to understand.

This is part 3 of FlyBy. Part 1 was a rate baseline. Part 2 was seeded loss. Here the workload is protocol_quotes: 10 kpps of binary market quotes for AAPL, one virtual second, no faults.

Why “bytes in a batch” is not enough

VirtualNic already paces packets. The interesting question is what sits in the payload. Built-in specs include fixed sequence numbers, random bytes, Gaussian sizes, custom callbacks—and PayloadSpec::Protocol.

The seed scenario uses ProtocolMessage::market_quote("AAPL"). The layout is 34 bytes, big-endian:

msg_type(u8)='Q' | flags(u8) | symbol([u8;8]) | bid(u64 BE) | ask(u64 BE) | seq(u64 BE)

Symbol is ASCII, padded with spaces to 8 bytes. Bid is 100_000 + seq % 1_000; ask is bid plus one. That is enough for a decoder to assert msg_type, a ticker, a spread, and a sequence.

There is also a length-prefixed FIX-like FixQuote (35=Q|55=…) if you want ASCII on the wire instead of the packed binary.

use flyby_simulator::{PayloadSpec, ProtocolMessage, TrafficConfig, TrafficPattern};

let quotes = TrafficConfig {
    pattern: TrafficPattern::FixedRate { pps: 10_000 },
    payload_size: 34,
    batch_size: 64,
    payload: PayloadSpec::Protocol(ProtocolMessage::market_quote("AAPL")),
};

The same choice appears in FlyScenario as [nic.payload] kind = "protocol" with proto = "market_quote" and a symbol.

Reproduce

cargo run -p flyby-simulator --bin flyby-sim -- protocol_quotes
./scripts/reproduce-article.sh part-vi-protocol-quotes
cargo run -p flyby-simulator --bin flyby-sim -- tui protocol_quotes

Approximate CLI (catalog expected output):

Running scenario 'protocol_quotes': 10 kpps binary market-quote payloads (AAPL).
  Payload  : Protocol(MarketQuote { ... })
  Note     : results are SIMULATED (not hardware)

Results (simulated):
  Packets generated : 10000
  Slots written     : 10000

Ten thousand generated and ten thousand slots written means the virtual shared-memory ring kept up for this rate and duration. That is a pipeline health check, not a market-data SLA.

What to test with this

  • A decoder that rejects msg_type != b'Q'.
  • Symbol padding (AAPL plus four spaces).
  • Monotonic seq in the last eight bytes.
  • Combine with part 2 so some quotes never arrive; the decoder should not assume a dense sequence.

Random payloads hide all of that. Protocol payloads make the simulator a peer of the production feed—same traits, smaller building.

Next: replaying a classic pcap of the same quote layout, captured rather than generated.